Reference is made to the following materials, the disclosures of which are hereby incorporated by reference herein:
[1] "The Measurement of Electrical Variables in a Submerged-Arc Furnace", Report No. 2093, National Institute for Metallurgy, South Africa, Apr. 15, 1981, 55 P. PA1 [2] Patent publications ZA 77/3923 and ZA 78/0375 (Republic of South Africa). PA1 [3] Heinonen, P., Neuvo, Y., "FIR-Median Hybrid Filters", IEEE Transactions on Acoustics, Speech and Signal Processing, vol. ASSP-35, June 1987, pp. 832-838. PA1 [4] Shepherd, W., Zand, P., Energy flow and power factor in nonsinusoidal systems, London, New York, Melbourne: Cambridge University Press, 1979.
So-called electrode furnaces can be categorized as (1) resistance furnaces, wherein electrodes are submerged in molten metal, (2) submerged-arc furnaces, wherein electrodes are submerged in a non-melting material, e.g., slag, and which are used for the reduction of metals etc., e.g., ferrochromium furnaces, ferromanganese furnaces, calcium-carbide furnaces, and (3) open-arc furnaces, wherein electrodes are only occasionally in contact with a material to be melted, e.g., scrap melting furnaces. For the sake of simplicity, this description only deals with a three-phase AC electric-arc furnace, but all the described aspects can be readily extended to cover any desired polyphase electrode furnace.
The open-arc and submerged-arc furnaces most commonly employ a so-called knapsack connection in which each line voltage is connected between two working electrodes; the total number of electrodes is three. Thus, the system does not include a neutral power connection. The advantage of the connection arrangement is that the electrode current is .sqroot.3 times the transformer secondary current. Since the electrode currents in a large-scale furnace exceed 100 kA, this represents a major advantage in transformer construction. Besides knapsack connected furnaces, this description deals with other AC electric-arc furnaces as well, including those with a neutral power connection.
The operation of several furnaces is inherently continuous: the furnace is charged and tapped while in operation. Erosion of the working electrodes is generally compensated for by adding blocks of self-baking carbon paste to the electrode tops and by slipping the electrode downwards according to its rate of erosion.
In order to optimize the operation of a furnace, the parameters of each working electrode must be adjusted individually. This is because the furnace does not operate in a homogeneous way: charging and tapping operations, for example, result in an uneven distribution of the charge. The adjustment operations include, e.g., manipulating the electrodes in the direction of the longitudinal axis, i.e., up and down, and adjusting the furnace transformer secondary voltages by the use of voltage tap changers. The purpose of the furnace control is to maximize the power factor and the active power delivered into the furnace and to eliminate the overload situations, such as surpassing the maximum current of the electrodes or the transformer secondary, and the apparent-power capacity of the transformer. On the other hand, another purpose of the control is to maintain an ideal reaction zone below the electrodes, which most often means that the same active power is supplied through all electrodes. Especially in high-capacity furnaces, the inductance of the furnace circuit in relation to the resistance is significant; as a result of this and the asymmetric state of the furnace, the relative active powers supplied by the electrodes may differ considerably from the ratios of electrode currents. Therefore, the measurement of the electrode-related active powers of the furnace is essential in view of the control. In a high-capacity furnace, there may occur a situation that the electrode-related reactance exceeds the corresponding resistance. Thus, increasing the current by reducing the resistance, i.e., by slipping the electrode, will reduce the active power supplied by the electrode. Therefore, it is very beneficial to know also the electrode-related resistances and reactances.
Since the arcing phenomenon occurring to a certain extent even in a submerged-arc furnace is not purely resistive but also includes a minor inductive component, the furnace active power in relation to the apparent power, i.e., the power factor, can be maximized by keeping the arcing phenomenon at its minimum and by maximizing the thermal power produced by resistive dissipation. The strength of arcing can be assessed, e.g., by measuring the amplitudes of distortion components of the electrode voltages. The distortion components of arc voltages can also be used for obtaining information about the operational phase of cyclically operating furnaces, e.g., scrap-mel ting furnaces.
The measuring information about power dissipated in the arc and the electrode-related active power or, on the other hand, the arc voltage, the electro de-related resistance and reactance are also utilized when predicting the erosion of electrodes. Even though prediction is used, the length of working electrodes must be periodically measured. The measurement can be most easily performed after stopping the furnace. However, many existing furnaces operate continuously and, thus, the length of electrodes can be readily measured only during operation halts. The fewer the opportunities of performing measurements, the more useful are the erosion models for prediction of the rate of the consumption of electrodes. For the above reasons, it is generally desirable either to measure the electrode-related distortion voltages or to measure directly the powers dissipated in the arc.
In the traditional measurement method, a measuring electrode, a so-called neutral connection, is placed in the carbon lining at the bottom of the furnace; this electrode is hopefully located at the real star point of the system. The furnace has no power return connection. The voltages per working electrode are measured relative to the neutral connection. There may even be three neutral connections, one for each working electrode. The furnace transformer secondary currents or electrode currents are measured by means of current transformers. If the furnace is in knapsack connection the current transformers of the furnace transformer may be wired in a delta-star arrangement so that the currents at the secondary are together directly proportional to the electrode currents.
The problems of the traditional measurement method include strong disturbances in voltage measurement and the fact that the process is not symmetrical, e.g., as a result of uneven charging, so that the neutral connection does not lie at the real star point of the system. The disturbance problem originates mainly from the electro-magnetic flux, caused by electrode and furnace currents and extending through a large loop formed by the measuring conductors. The considerable loop size results from the fact that the objects to be measured, the top portions of working electrodes and, on the other hand, the neutral connection(s), are separated from each other by a distance of several meters. The neutral connection can also be easily broken in the hot environment and replacement of the connection is very difficult if the breakage is inside the furnace shell. In view of disturbances, a measuring system provided with three neutral connections is a substantial improvement if the voltage measuring leads are routed in the best possible way.
In view of disturbances and reliability a substantially improved system has been developed on the basis of the assumption that proportions of the inductances of secondary circuits supplying a furnace are known and remain constant or at least are calculable and that the fluctuation of furnace power is primarily caused by the fluctuations of resistances. When applying this assumption, a furnace neutral connection is not required and, thus, the resistances of the secondary circuits can also be measured from the primary of the furnace transformer. However, besides resistance, the electric arc contains some inductance (due to the strong effect of temperature on the conductivity of gas plasma and to thermal time constants), and also the length and position of electrodes have an effect on electrode-related inductances, so the above assumption of the permanently constant or calculable proportion of inductances is not strictly valid. Neither does the method account for the strong non-linearity of a furnace (and the electric arc). This is probably the reason why the method has not been applied to measurement of the distortion components of electrode-related voltages and, thus, the estimation of power produced by the electric arc must be performed by using a measuring system provided with neutral connection(s) at the bottom of the furnace.